This invention related to infrared cameras, and more particularly to a room temperature infrared camera in which matched photoresistors are used in a differential mode to null out ambient-induced dark current.
It will be appreciated that in a military context it is important to be able to identify an incoming missile due to its IR signature. The infrared signature must be discriminated from the background such that the missile is discernible from, for instance, a rock or sand dune which can reflect ambient sun light into the sensor aperture. In the past, wide angle infrared cameras have been proposed in which infrared cameras are placed about the tank so that incoming missiles can be spotted from any direction.
In order to accomplish robust target detection, it was thought that the only way to be able to differentiate the IR signature of an incoming missile from the background was to employ cryogenically-cooled detectors located in vacuum bottles so that the radiation from the target could be distinguished from background. Also it was thought that it would be necessary to provide a reference sensor and chop its output to provide a uniform background reference against which to compare IR detector outputs.
Also, in the past IR detectors have been made from single crystals such as InSb, and HgCdTe which respond to various red spike and blue spike characteristics of the incoming missile. Single crystal detectors require cryogenic cooling to meet sensor sensitivity requirements and tend to be too expensive for ground-vehicle self protection applications.
Note that, the purpose of using red spike and blue spike detection algorithms is to be able to distinguish the 4 micron missile radiation from radiation attributable to the surrounding background and solar reflections. Blue spike detection refers to detection in the 3.8-4 micron range, whereas red spike detection is usually in the 4.4-4.7 micron range.
In short, in the past cryogenically-cooled detectors with chopped reference signals have been tuned to the red spike and blue spike ranges in order to be able to distinguish an IR target from the surrounding terrain.
It will be appreciated that aside from the difficulty of providing a stable reference, the cryogenic and vacuum bottle requirements are such that infrared cameras have only been achievable at great cost. Not only are the cameras expensive due to the packaging required for the cryogenically-cooled thermos bottles in which an ultra high vacuum must be maintained and for which cryogenic refrigerators must be employed, maintenance for such a camera in the field is a large problem.
With a tank having 4 to 5 scanning infrared cameras, the cost of such an array of cameras plus processing makes such a non-room temperature solution impractical. Additionally, when field maintenance is added to the cost, the reliable cost-effective infrared camera is not presently available for use in the military arena.
In the civilian context, it is exceedingly desirable to have a wide angle infrared camera to be able to scan the output of, for instance, a sluice gate in a paper making machine to detect the degree of moisture in the sheet as it is being dried over the Fourdrinier wires. In the past, single pixel infrared detectors have been utilized to determine the moisture content of the web that is being dried.
However, the single point detectors do not take into account the fact that the web varied in moisture across its lateral extent. Controlling the rate at which paper is made based on a single infrared detector does not take into account the fact that the web itself is not uniform. It is therefore desirable to provide a wide angle infrared camera to detect the moisture content across the entire sheet to provide more precise process control.
It will thus be appreciated that infrared cameras operating in hostile environments are required in a variety of different applications such as steel making, and other processes such as plastic injection molding and pollution monitoring. In addition, such a camera could be employed for non-invasive glucose monitoring for diabetic patients.
Rather than providing a wide angle infrared camera with cryogenic cooling and vacuum apparatus, in the subject invention a room temperature wide angle infrared camera is provided through the utilization of photoresistors which have marked linearity in their response. The linearity stems from their polycrystalline nature and large boundary size. These photoresistors are located side by side, with the one photoresistor being the active photoresistor and with the other photoresistor being the xe2x80x9cblindxe2x80x9d photoresistor. As a result, the photoresistor elements are subjected to the same ambient temperature, with the so called xe2x80x9cblindxe2x80x9d photoresistor having its output subtracted from that of the active photoresistor to provide a differential output. This differential output is then utilized as the target indicating signal, with the ambient having been subtracted out at the photoresistor array. Note that, because of the linearity of the photoresistors, no chopped references or temperature stabilizers are required. The only thing that may be required is a thermo-electric heat sink at the back of the array to keep it at or near room temperature.
In one embodiment, accurate dark current nulling is provided through the utilization of a xe2x80x9cblindxe2x80x9d polycrystalline lead salt resistor matched to an active polycrystalline lead salt photoresistor. These devices exhibit a high degree of linearity over varying ambient temperatures that permits exact cancellation of the ambient due to the matched characteristics of the active photoresistor and its reference photoresistor, while at the same time providing a useable output to distinguish infrared target radiation from the surrounding terrain.
In one embodiment infrared photoresistors are tailored to two different wavelengths corresponding to the aforementioned red spike and blue spike by providing photoresistors having different lead salt compositions. To this end, a compact two color array is provided by stacking the photoresistors responding to a first wavelength on top of those responding to a second wavelength, with each set of photoresistors having companion and matching xe2x80x9cblindxe2x80x9d photoresistors to permit dark current cancellation.
Not only is this room temperature infrared camera of use in military applications, it may also be used for process control where the temperature of a wide expanse of material must be monitored. Papermaking and steelmaking are two examples.
It is even possible to overlay the array with a variable spectral filter so that different columns of the display can monitor different wavelengths thus to generate a temperature profile.
In summary, a high resolution room temperature infrared camera requires no cryogenic cooling and high vacuum packaging, normally thought necessary for infrared target detection, by using a xe2x80x9cblindxe2x80x9d polycrystalline lead salt resistor in combination with a matched active polycrystalline lead salt photoresistor both maintained at room temperature to provide a differential output indicative of an IR generating target. As a result of the matched components, the differential output nulls out the dark current which is the result of the ambient at the camera. The high degree of linearity associated with polycrystalline photoresistors permits fabrication of a differential detector where dark current in the active element is nulled out by equal and opposite current flow in a non-optically active reference resistor. Rather than a reference provided by a mechanical chopper normally used to correct for array non-uniformity, and costly temperature stabilization, the subject polycrystalline photoresistor provides a factory-setable reference, such that dark current can be canceled with the use of a nearly identical xe2x80x9cblindxe2x80x9d element beside the optically active element.